Mismatch brain response to speech sound changes in rats

Mustak Ahmed

doi:10.3389/fpsyg.2011.00283

Abstract

Understanding speech is based on neural representations of individual speech sounds. In humans, such representations are capable of supporting an automatic and memory-based mechanism for auditory change detection, as reflected by the mismatch negativity (MMN) of event-related potentials. There are also findings of neural representations of speech sounds in animals, but it is not known whether these representations can support the change detection mechanism analogous to that underlying the MMN in humans. To this end, we presented synthesized spoken syllables to urethane-anesthetized rats while local field potentials were epidurally recorded above their primary auditory cortex. In an oddball condition, a deviant stimulus /ga/ or /ba/ (probability 1:12 for each) was rarely and randomly interspersed between frequently presented standard stimulus /da/ (probability 10:12). In an equiprobable condition, 12 syllables, including /da/, /ga/, and /ba/, were presented in a random order (probability 1:12 for each). We found evoked responses of higher amplitude to the deviant /ba/, albeit not to /ga/, relative to the standard /da/ in the oddball condition. Furthermore, the responses to /ba/ were higher in amplitude in the oddball condition than in the equiprobable condition. The findings suggest that anesthetized rat’s brain can form representations of human speech sounds, and that these representations can support the memory-based change detection mechanism analogous to that underlying the MMN in humans. Our findings show a striking parallel in speech processing between humans and rodents and may thus pave the way for feasible animal models of memory-based change detection.

Highlights

The ability to represent individual speech sounds is a necessary condition for understanding speech
There are findings of neural representations of speech sounds in animals, but it is not known whether these representations can support the change detection mechanism analogous to that underlying the mismatch negativity (MMN) in humans
The present study addresses whether brains of urethaneanesthetized rats generate local field potentials functionally analogous to human MMN to speech sounds

Summary

Introduction

The ability to represent individual speech sounds is a necessary condition for understanding speech. This ability is not unique to humans. Neural representations of speech sounds are formed already pre-attentively. They can support automatic detection of changes, as reflected by a component of event-related potentials of the brain named mismatch negativity (MMN, Näätänen et al, 1978, for a review, see Näätänen et al, 2007). The disappearance of MMN in control conditions in which the standard stimuli are removed (so called deviant-alone condition or equiprobable condition, e.g., Alho et al, 1990; Jacobsen and Schröger, 2001) suggests that standard stimuli form a memory trace against which incoming sounds are compared in the neural level. The mismatch response is elicited when discrepancy between the memory representation and the deviant input is detected (memory-comparison explanation of MMN: Näätänen, 1990; Näätänen et al, 2005)

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Conclusion